linux-kernel - Re: [PATCH v9 07/18] arm64: trans_pgd: hibernate: idmap the single page that holds the copy page routines

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Date:   Wed, 29 Apr 2020 18:01:19 +0100
From:   James Morse <james.morse@....com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     jmorris@...ei.org, sashal@...nel.org, ebiederm@...ssion.com,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        corbet@....net, catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, maz@...nel.org,
        vladimir.murzin@....com, matthias.bgg@...il.com,
        bhsharma@...hat.com, linux-mm@...ck.org, mark.rutland@....com,
        steve.capper@....com, rfontana@...hat.com, tglx@...utronix.de,
        selindag@...il.com
Subject: Re: [PATCH v9 07/18] arm64: trans_pgd: hibernate: idmap the single
 page that holds the copy page routines

Hi Pavel,

On 26/03/2020 03:24, Pavel Tatashin wrote:
> From: James Morse <james.morse@....com>
> 
> To resume from hibernate, the contents of memory are restored from
> the swap image. This may overwrite any page, including the running
> kernel and its page tables.
> 
> Hibernate copies the code it uses to do the restore into a single
> page that it knows won't be overwritten, and maps it with page tables
> built from pages that won't be overwritten.
> 
> Today the address it uses for this mapping is arbitrary, but to allow
> kexec to reuse this code, it needs to be idmapped. To idmap the page
> we must avoid the kernel helpers that have VA_BITS baked in.
> 
> Convert create_single_mapping() to take a single PA, and idmap it.
> The page tables are built in the reverse order to normal using
> pfn_pte() to stir in any bits between 52:48. T0SZ is always increased
> to cover 48bits, or 52 if the copy code has bits 52:48 in its PA.
> 
> Pasha: The original patch from James
> inux-arm-kernel/20200115143322.214247-4-james.morse@....com

-EBROKENLINK

The convention is to use a 'Link:' tag in the signed-off area.
e.g. 5a3577039cbe

> Adopted it to trans_pgd, so it can be commonly used by both Kexec
> and Hibernate. Some minor clean-ups.

Please describe your changes just before your SoB. This means each author sign's off on
the stuff above their SoB, and its obvious who made which changes.

Search for 'Lucky K Maintainer' in process/submitting-patches.rst for an example.


> diff --git a/arch/arm64/include/asm/trans_pgd.h b/arch/arm64/include/asm/trans_pgd.h
> index 97a7ea73b289..4912d3caf0ca 100644
> --- a/arch/arm64/include/asm/trans_pgd.h
> +++ b/arch/arm64/include/asm/trans_pgd.h
> @@ -32,4 +32,7 @@ int trans_pgd_create_copy(struct trans_pgd_info *info, pgd_t **trans_pgd,
>  int trans_pgd_map_page(struct trans_pgd_info *info, pgd_t *trans_pgd,
>  		       void *page, unsigned long dst_addr, pgprot_t pgprot);

This trans_pgd_map_page() used to be create_single_mapping(), which is where the original
patch made its changes.

You should only need one of these, not both.


> +int trans_pgd_idmap_page(struct trans_pgd_info *info, phys_addr_t *trans_ttbr0,
> +			 unsigned long *t0sz, void *page);
> +
>  #endif /* _ASM_TRANS_TABLE_H */

> diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c
> index 37d7d1c60f65..c2517d1af2af 100644
> --- a/arch/arm64/mm/trans_pgd.c
> +++ b/arch/arm64/mm/trans_pgd.c
> @@ -242,3 +242,52 @@ int trans_pgd_map_page(struct trans_pgd_info *info, pgd_t *trans_pgd,
>  
>  	return 0;
>  }
> +
> +/*
> + * The page we want to idmap may be outside the range covered by VA_BITS that
> + * can be built using the kernel's p?d_populate() helpers. As a one off, for a
> + * single page, we build these page tables bottom up and just assume that will
> + * need the maximum T0SZ.
> + *
> + * Returns 0 on success, and -ENOMEM on failure.
> + * On success trans_ttbr0 contains page table with idmapped page, t0sz is set to

> + * maxumum T0SZ for this page.

maxumum

> + */


Thanks,

James